What is a sea cucumber
What is a sea cucumber

What is a sea cucumber? if you have ever been scuba diving or snorkeling you likely came across turd-like sausages on the ocean floor – Sea cucumbers.

Despite their name and their motionless lifestyle, they are animals.

And although they look like a weird sea version of slugs, worms, or millipedes. they are actually closer related to us humans.

Their closest relatives, however, are starfish, sea urchins, and sand dollars with which they form the Echinoderms.

Despite Sea cucumbers looking intrinsically harmless, they are actually quite the opposite

if you aren’t careful.

In this article, we will discuss why that is as well as everything else that makes these little turds interesting.

Enjoy a defining trait of Echinoderms is their radial symmetry consisting of usually 5 mostly identical body parts.

what is a sea cucumber-5 mostly identical body
what is a sea cucumber-5 mostly identical body?

You can find this symmetry in sea cucumbers too but it’s far less obvious than in sand dollars or starfish for instance.

If you were to stand a sea cucumber upright on its mouth or its anus you can however still see the symmetry in the 5 body segments that connect these two points – at least in some species.

So in comparison to starfish and sea urchins, sea cucumbers essentially lay on their sides.

Because of this, they have also evolved a weak bilateral symmetry as their individual segments have developed differently.

what is a sea cucumber?-bilateral symmetry
what is a sea cucumber?-bilateral symmetry

3 of them form the trivium, the food so to speak.

While the 2 other forms are the bivium, the dorsal face.

Each of the segments has a row of tube feet, similar to those you can find on the underside of starfish, that allow the animal to crawl along.

sea cucumber anatomy
sea cucumber anatomy

On the dorsal face, these tube feeds are often under-developed or are lacking altogether.

Other species have repurposed them as a form of protection or as sensory organs.

The ring of feeding tentacles around the mouth are similarly modified and enlarged tube feet.

sea cucumber feeding tentacles
sea cucumber feeding tentacles

These can be retracted into the body by a set of retractor muscles when they feel threatened.

Additionally, 5 lengthwise running bands of muscles allow the sea cucumber to expand and contract its body.

Sea cucumbers don’t have a brain.

instead, they have a simple system of nerve strings that extend from a nerve ring near the mouth, the central hub of the sea cucumbers anatomy. and run longitudinally along the walls.

sea cucumber central nerve ring
sea cucumber central nerve ring

With this rudimentary nervous system, they are able to detect touch and chemical stimuli and coordinate all their bodily functions including their movement.

Responsible for locomotion in sea cucumbers like in all echinoderms is the water vascular system, a system unique in the animal kingdom, that works almost entirely hydraulically, so through water pressure.

In starfish, part of this system is a sieve-like orifice on the upper face of the animal through which they can take in seawater.

In sea cucumber, however, this sieve is regressed making their water vascular system a closed circuit.

Other than that they work almost entirely the same.

From the central ring canal, 5 rows of canals branch out to each row of tube feet.

What is a sea cucumber?

Each of these canals is connected to hundreds of ampullae, little sack-like structures that expand when water from the radial canal is pumped into them.

Each ampulla is linked to a single tube foot.

Through contraction of the sac, the sea cucumber forces water into the tube foot, causing it to extend and push against the ground.

The minute muscle controls the direction in which it points.

Similarly, a relaxation of the sac makes the foot retract again.

Through hundreds of these little feet, individually but coordinately controlled, sea cucumbers are able to crawl along in any direction, but only very slowly.

Similarly, spectacular is their respiratory system because sea cucumbers essentially breathe through their butts.

Their main respiratory organs are two trees of hollow highly branched tubes, called water lungs.

These trees are attached to the cloaca of all things.

sea cucumber water lungs and cloaca
sea cucumber water lungs and cloaca

By closing their anal sphincter muscle they can force the water into the system, allowing them to breathe even more absurd is that some animals, after analyzing their options, decided that this is the best place to live, inside the cloak of sea cucumbers.

A variety of fish, most commonly pearlfish, as well as a few species of scale worms, and also some crabs use the anal cavity of certain sea cucumbers for protection from predators.

These are so-called commensalism symbiotic relationships, so relationships that as opposed to parasitism, or mutualism benefit one side and down negatively impact the other.

Still, some species of sea cucumbers have developed anal teeth, presumed to keep the unwanted house guests out.

Sea cucumbers can be found in every ocean around the world, in shallow waters as well as the deep sea, down to depths of up to 10km. on the deep seafloor, they make up the majority of the animal biomass, up to 95% in some places.

Here giant herds of sea cucumbers, consisting of millions of animals wanter across the ocean floor in search of food.

In addition to their numbers, they are also surprisingly diverse.

Over 1700 species are known today.

The biggest of them (Synapta maculata) can reach a length of up to 3m, the smallest measure only a few millimeters (Rhabdomolgus ruber).

Typically however, they are between 10-30cm long.

Sea cucumbers feed on detritus and debris, algae, and plankton.

Essentially everything that ends up in their mouths and is useable gets processed.

They typically use one of two strategies to acquire their food.

Some species are filter-feeders.

These usually have developed large feathery tentacles that they flail in the water column to catch free-floating plankton.

But most of them are sediment feeder that uses their tentacles more like shovels to fill up on mud on sand from the ocean floor.

These materials then make their way through their intestines while the organic matter in them is digested.

During this process, the organic particles are broken down into far tinier pieces, which are then excreted together with the undigestable sand and will become fodder for even smaller lifeforms.

So similarly to earthworms in terrestrial ecosystems sea cucumbers by filtering sediments and recycling nutrients back into the food web, play a major role in the biological processing of the seabed.

A single specimen can digest more than 45 kg of sediment a year.

Legend has it that every grain of sand in the ocean has at some point or another been excreted by a sea cucumber.

Even their reproduction seems perfectly fitting for their crude lifestyle.

Instead of going through the trouble of dating, they simply release both eggs and sperm into the surrounding water hoping that they meet.

To maximize fertilization success this usually happens simultaneously in an area.

Sea cucumbers are also notable because they possess diverse regeneration abilities.

Some species have the ability to recover from the loss of 50% or more of their body which they sometimes use deliberately to reproduce asexually.

Basically, the sea cucumber splits itself in half and each half regrows into a full sea cucumber.

Other echinoderms such as Starfish have similarly the ability to regrow lost body parts.

Some can regrow limbs and a few can even regrow a completely new body from a single arm even from an arm fragment just 1cm long.

As cool as this is, it doesn’t change the fact that on the face of it sea cucumbers are quite benign animals and seemingly easy prey for predators.

However, the majority of them have, despite their apparent helplessness, developed effective defense mechanisms. many of them use the most obvious method of protection for such an animal, toxin.

Most species carry a toxin called Holothurin in their organs, some can even release it through their skin when they feel threatened.

sea cucumber holothurin
sea cucumber holothurin

This toxin is so effective in killing small animals particularly fish that it was even used by fishermen from indo-pacific regions to hunt fish.

Most sea cucumbers have in addition to that the absurd ability to expel their inner organs from their butts sometimes from their mouths and discharge them into the surrounding water.

This is called evisceration and is meant to distract or scare predators away.

As brutal as this seems this behavior does not kill the sea cucumber.

As mentioned earlier they have incredible self-healing capabilities, which allow them to simply regenerates the organs in the following weeks.


A few species have even developed a special organ for that purpose, the so-called Cuvierian tubules.

sea cucumber cuvierian tubules
sea cucumber cuvierian tubules

These are long white tubes that are attached to one, or both, respiratory trees.

If the cucumber is attacked by a predator it will expel these sticky, stringy sometimes toxic threads out of its anus.

The resulting net of strings can entangle immobilize and even kill potential predators and gives the sea cucumber the opportunity to crawl away.

This is yet another way these sedentary animals can defend themselves.

As a result of these multifaceted defense mechanisms, sea cucumbers are typically avoided by generalist predators.

A few highly specialized ones, however, do prey on them.

Among them are two species of sea snails.

The giant tun and the partridge tun.

These basketball-sized snails are immune to the sea cucumbers toxin, they have, however, their own very potent poison that they use to paralyze the sea cucumber.

Because I guess when you are a snail, a sea cucumber running away from you is a real concern.

Paralyzed and with their toxins not affecting the snail the sea cucumber then has to experience being swallowed whole and alive.

This concludes the most boring hunt in the animal kingdom.

For humans touching sea cucumbers is aside from allergic reactions to their toxin generally not life-threatening.

The contact with their toxic threads, outer skin, or ejected innards can, however, cause sometimes painful skin irritations that may be similar to jellyfish stings.

If the toxins come into contact with your eyes they can even cause blindness when left untreated.

So as long as you don’t know what you’re doing and what kind of sea cucumber, you have in front of you You’re probably best advised to leave them be.

Needlessly stressing wild animals by touching and picking them up, is a douchy thing to do anyway. just watch them from afar.

That being said, not every sea cucumber species is poisonous.

The ones that aren’t and even a few that are, are often considered a delicacy in many places, particularly in Asia in countries such as China and Japan.

In Japan, the history of sea cucumber fishery dates back over 1000 years to the 8th Century.

Here sea cucumbers are traditionally served in the form of a soup.

But this too isn’t always safe.

It can happen that species get confused or meals aren’t prepared correctly.

When that happens and you accidentally eat a poisonous sea cucumber the effects are a lot worse.

It can cause severe abdominal pains and vomiting, numbness, and breathing difficulties.

A few people have even died after eating poisonous sea cucumber.

But given how many tons of sea cucumber are eaten every year these cases are extremely rare.

Because of an ever-increasing demand over the past few decades particularly in China substantial numbers of sea cucumbers are now bred and cultured artificially in seawater ponds or sea ranches.

Depending on the region is takes between 2-6 years for an individual to grow to a marketable size.

The Japanese sea cucumber is the most important and valuable commercial sea cucumber species and China is by far its biggest producer.

Their Total production amounted to 200 000 tonnes in 2017.

This makes sea cucumber farming a multi-billion dollar industry for China(4.44 billion).

China sea cucumber sales
China sea cucumber sales

But not every sea cucumber was content with their lives as the vacuum cleaners of the oceans.

Some of them have developed bodily features that allowed them to lift themselves up from the ocean floor and take to the skies.

26 of these swimming sea cucumbers are currently known but it is suspected that there are more of them.

Most of these inhabit the deep sea and are incredibly rare, which is why it is understandably difficult for scientists to shed a light on this remarkable and elusive subset of sea cucumbers

The most famous of them is probably the fittingly dubbed headless chicken monster.

These equally beautiful and bizarre-looking sea cucumbers have repurposed some of their tube feet and developed them into three webbed veil-like sails.

Like their ground-bound relatives, these sea cucumbers are sediment feeders.

In between meals, however, they spend most of their time swimming, using their sails to sometimes rise as much as 1km above the ocean floor.

This allows them to avoid ground-living predators and to find better feeding grounds.

Pelagothuria matrix another species of swimming sea cucumber takes this way of life to the extreme.

They have left their ground-living lifestyle behind completely.

Unlike the previous example, they spend their entire living off the seafloor using their feeding tenticles on top of their almost circular umbrella to catch plankton and algae.

They are the only known sea cucumber even the only known echinoderm that conquered the open ocean.

All this makes sea cucumbers truly amazing little animals that even after more than 350 million years on this planet are still going strong and as successful as ever.

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